This invention relates to a vehicle parking brake system.
Typically, the parking brake function of a vehicle takes the form of a manually operated brake in which the amount of parking brake pressure is regulated by the vehicle operator. When this form of parking brake is operated while the vehicle is moving, excessive slip of the braked wheels leading to a wheel lockup condition may be avoided by the vehicle operator by consciously avoiding application of parking brake pressure giving rise to an excessive slip condition. However, this is difficult in view of the fact that when the critical slip value producing a peak braking effort is exceeded, the braked wheel rapidly decelerates toward lockup.
Recently, it has been proposed to automatically apply the parking brake in response to the actuation of a switch by the vehicle operator. In one such system, the brake pressure is controlled, when the switch is operated while the vehicle is moving, to establish a target vehicle deceleration until the vehicle stops after which the brake pressure is increased to maintain maximum braking force. In this form of parking brake system, an excessive slip condition will result if the target vehicle deceleration cannot be achieved on the particular road surface. For example, if the road surface coefficient of friction is low such as when the road surface is covered with ice, the maximum achievable vehicle deceleration is low. If the target vehicle deceleration is greater than this maximum possible vehicle deceleration, the brake pressure controlled in an attempt to achieve this target vehicle deceleration will result in the critical slip being exceeded and the wheel being decelerated toward lockup. To avoid this condition by setting a low target vehicle deceleration results in a lower than possible deceleration on higher coefficient of friction surfaces.